Tetrakis(fluoroaryl)borate derivatives are useful compounds as, for example, a co-catalyst for promoting the activity of a metallocene catalyst (polymerization catalyst) for a cationic complex polymerization reaction and a catalyst for photopolymerization of silicone. Moreover, tetrakis(fluoroaryl)borate compounds are compounds which are useful as intermediates for the preparation of the tetrakis(fluoroaryl)borate derivatives. In resent years, the metallocene catalyst is particularly noted as a catalyst for polymerization of polyolefin.
For instance, J. Organometal. Chem., 2 (1964), pp.245-250 discloses a method for synthesizing a tetrakis(pentafluorophenyl)borate derivative by reacting bromopenetafluorobenzene with butyl lithium (an organic lithium compound) at -78.degree. C. with the use of dry pentane as a solvent to form pentafluorophenyl lithium, reacting this compound with a boron trichloride (halogenated boron) to synthesize a tris(pentaflourophenyl)borane, and reacting this compound with pentafluorophenyl lithium to synthesize the tetrakis(pentafluorophenyl)borate derivative. Moreover, Tokukaihei No. 6-247981 (Japanese laid-open patent application, published Sep. 6, 1994) discloses a method for synthesizing a tetrakis(pentafluorophenyl)borate-lithium that is a kind of tetrakis(fluoroaryl)borate compound from pentafluorobenzene with the use of an organic lithium compound and halogenated boron.
However, these methods suffer from the problems: 1) since the organic lithium compound is unstable with respect to heat, the temperature of the reaction system must be maintained at a temperature of not higher than -65.degree. C., and therefore special facilities are required and it takes a cost for cooling; 2) since an expensive organic lithium compound must be used and ignition may occur by the reaction between the compound and water, etc, it is dangerous to handle the compound; and 3) since expensive halogenated boron must be used and the compound is in gaseous phase and corrosive, the compound is difficult to handle. It is thus difficult to industrially implement the method disclosed in the above-mentioned publications.
In contrast, for example, U.S. Pat. No. 5,473,036 discloses a method for preparing triethylammonium*tetrakis (pentafluorophenyl)borate by reacting bromopentafluorobenzene with magnesium to give pentafluorophenyl magnesium bromide, reacting this compound as a Grignard reagent with a boron trifluoride diethyl ether complex to synthesize tetrakis (pentafluorophenyl) borate magnesium bromide, and reacting this compound with triethylammonium-chloride to prepare the triethylammonium tetrakis(pentafluorophenyl) borate. Further, Tokukaihei No. 6-247980 (Japanese laid-open patent application, published Sep. 6, 1994) discloses a method for synthesizing a tetrakis(fluoroaryl)borate compound by a Grignard reaction, and more specifically a) a method for preparing a tetrakis(pentafluorophenyl)borate-magnesium halide by reacting a pentafluorophenyl magnesium halide with halogenated boron; and b) a method for preparing a tetrakis(pentafluorophenyl)borate magnesium halide by reacting a pentafluorophenyl magnesium halide with tris(pentafluorophenyl) boron.
In the preparation method of United States Patent No. 5,473,036, magnesium bromide fluoride and magnesium chloride fluoride are given as by-products together with triethylammonium tetrakis(pentafluorophenyl)borate. However, this publication does not disclose or mention separation and removal of the halogenated magnesium as the by-product from the reaction system. Since triethylammonium tetrakis(pentafluorophenyl)borate is a crude product, in order to obtain a final purified product, it is necessary to recrystallize the compound with the use of chloroform or methylene chloride/butyl ether. It is thus hard to say that the above preparation method is an industrially effective method.
Besides, in the above-mentioned preparation method a) of Tokukaihei No. 6-247980, halogenated magnesium is given as a by-product together with the tetrakis(pentafluorophenyl)borate. magnesium halide. However, this publication does not disclose or mention separation and removal of the halogenated magnesium as the by-products from the reaction system. When a tetrakis(pentafluorophenyl)borate derivative which is prepared with the use of tetrakis(pentafluorophenyl)borate-magnesium bromide containing halogenated magnesium as impurities is used as, for example, a co-catalyst of a metallocene catalyst, the activity of the catalyst is considerably owered. Therefore, the tetrakis(pentafluorophenyl)borate magnesium bromide prepared by this method is not suitable as the intermediate for the preparation of the tetrakis(pentafluorophenyl)borate derivative.
The pentafluorophenyl magnesium halide as a Grignard reagent used in the above U.S. Pat. No. 5,473,036 and Tokukaihei No. 6-247980 and more specifically, for example, pentafluorophenyl magnesium bromide formed from bromopentafluorobenzene and magnesium, is colored black by colored components as impurities produced by a side reaction, etc. Therefore, a tetrakis(fluoroaryl)borate compound synthesized with the use of the pentafluorophenyl magnesium halide is colored black by the colored components unless the step of removing the colored components is executed. Thus, when a tetrakis(fluoroaryl)borate derivative as an object is prepared using the tetrakis(fluoroaryl)borate compound, the tetrakis(fluoroaryl)borate derivative has a bad color tone. Hence, it is difficult to provide tetrakis(fluoroaryl)borate derivatives as products.
In short, the tetrakis(fluoroaryl)borate compound containing the colored components as impurities is not suitable as the intermediate for the preparation of a tetrakis(fluoroaryl)borate derivative. However, the above-mentioned publications do not disclose or mention the removal of the colored components.
Therefore, there is a great demand for a purifying method capable of efficiently and easily separating and removing colored components contained as impurities in tetrakis(fluoroaryl)borate compounds. There is also a great demand for a method capable of preparing a highly-pure tetrakis(fluoroaryl)borate derivative efficiently and at low costs.